Method of coagulating sludge

Abstract

 A method of coagulating sludge, which comprises adding sequentially an anionic polymer coagulant and a cationic polymer coagulant to sludge.

Description

[0001] The present invention relates to a novel method of coagulating sludge. More particularly it relates to a method which permits settlement and coagulation of sludge produced from a river, a lake or an engineering work site with high efficiency to largely facilitate dewatering and solidification.

[0002] Various methods have conventionally been tried to treat contaminated, sludge-containing water or water having sludge therein e.g. floating on a river or a lake, or sludge resulting from engineering works. The usual practice for treating such sludge is to cause it to settle by addition of an inorganic or polymer coagulant or a supplementary reagent, and to dewater and solidify the resultant coagulum for rejection or re-use.

[0003] However, these conventional methods have not been successful in causing highly efficient settlement and coagulation of sludge at a low cost.

[0004] Many further coagulants and supplementary reagents have been developed and methods of their application have been tested, but it has not yet been possible to conduct an industrially large-scale treatment at a low cost.

[0005] In the conventional sludge treatment methods, sludge to be covered has been believed to be negatively charged in general, so that a practice has been adopted of adding a cationic inorganic salt such as iron or aluminium, or a cationic polymer coagulant, to cause coagulation of sludge particles into flocks, and then adding a supplementary anionic reagent to cause settlement of the flocks.

[0006] However, whichever cationic coagulant or anionic reagent is used in the conventional methods, (rejection) separation and disposal of the large amount of flocks having a high water content, has required a mechanical dewatering by means of filter press or a belt press. In the absence of this mechanical dewatering, this disposal coagulation has required large areas of land, solidification requiring a long period of time, and even once solidified, rainfall may have cause swelling of the material to retard the procedure.

[0007] A further problem with conventional methods is that the use of an inorganic salt renders the pH of the supernatant liquid acidic, so that the necessary neutralization thereof adds a considerable cost.

[0008] We have surprisingly found that it is very effective to first add an anionic polymer coagulant to the sludge, and then to add a cationic polymer coagulant. This is in contrast to the conventional and general understanding of the mechanism of the procedure. Our belief is that settlement and coagulation of sludge are caused, not by a simple ionic interaction of positive and negative ions, but by a complicated mechanism in a colloidal state.

[0009] The present invention, on the basis of the above-mentioned finding, therefore seeks to provide a novel method of coagulating sludge, which permits an improvement over conventional methods, settlement and coagulation of sludge at a high efficiency and lower cost, and will lead to a considerable reduction of the burden of mechanical dewatering and solidifcation.

[0010] In particular the present invention provides a new method for using polymer coagulants which is contrary to conventional practice and teaching, and a method of coagulating sludge permitting a large-­scale treatment at a high efficiency and a lower cost with the use of such a new method.

[0011] These and other aspects and advantages of the invention will become more apparent in the detailed description and examples which follow.

[0012] According to one aspect of the invention, we provide a method of coagulating sludge, which comprises adding sequentially to sludge an anionic polymer coagulant and a cationic polymer coagulant. This may be followed by an additional anionic polymer coagulant.

[0013] In consequence of the procedure to be followed, and assuming that the sludge is negatively (-) charged, the present invention comprises adding a first anionic polymer coagulant to the sludge to increase the negative charge of sludge as a whole, and then adding a cationic polymer coagulant to rapidly accelerate the settlement of flocks. Addition of the first anionic polymer coagulant causes the state of charge of sludge particles and the colloidal state to change, and the subsequent addition of the cationic polymer coagulant will allow settlement in bulk.

[0014] When the supernatant liquid shows a somewhat high turbidity due to the addition of the cationic polymer coagulant, addition of a further anionic coagulant permits complete settlement, thus giving a clear supernatant liquid.

[0015] The most suitable type of coagulant will depend inter alia, on its affinity towards the sludge in question, and cost. There is therefore no limit on which polymer coagulants may be used but typically, any such anionic polymer coagulant will preferably comprise a 0.1% pure aqueous solution with a pH of from 6 to 8, and a viscosity of at least 100 cp (25°C, a B-type rotary viscosimeter, 30 rpm). A preferred example is an acrylamide polymer coagulant. For the cationic coagulant, any polymer coagulant with a pH of from 3 to 6 such as an ester coagulant may be employed.

[0016] The amount of coagulant to be added, varies with the treatment specifications of each batch, sludge concentration, or whether there is to be a continuous process. However there may be about 1 to 4% total of all coagulants added relative to the volume of material to be treated on the assumption of a sludge concentration of about 3 to 5%. This is not of course a limitative standard. (18 cc) and a cationic polymer coagulant (12 cc) were added in this order. As the anionic polymer coagulant, an acrylamide polymer coagulant (acrylamide acrylic acid co-polymer) with a pH of from 6 to 8 and a viscosity of at least 140 cp was used. As the cationic polymer coagulant, an ester polymer coagulant (poly-aminoalkyl metacrylate) having a pH of from 3 to 5 and a viscosity of at least 20 cp was employed.

[0017] In the comparison Examples, the same cationic (inorganic metal salt;AlCl₃) coagulant (6 cc) and then the same anionic polymer coagulant (16 cc) were added in this sequence.

[0018] Values of the amount of settlement (cc) after the lapse of five minutes and those of the water content (%) after 24 hours dewatering are shown in Table 1. As is clear from Examples 1 to 3 of the present invention, sludge treated in accordance with the present invention has an excellent settlement property and degree of dewatering. Settlement and coagulation of the sludge are rapid, and the remaining water content is significantly lower.


were carried out on sludge-bearing water from lakes and rivers in three localities (A), (B) and (C) in the Kanto area (Japan).

[0019] The 1000 cc of test-sludge for each settlement test had a concentration of 3% and was conducted in a 1000 cc glass cylinder.

[0020] In Examples 1 to 3, an anionic polymer coagulant (18 cc) and a cationic polymer coagulant (12 cc) were added in this order. As the anionic polymer coagulant, an acrylamide polymer coagulant (acrylamide acrylic acid co-polymer) with a pH of from 6 to 8 and a viscosity of at least 140 cp was used. As the cationic polymer coagulant, an ester polymer coagulant (poly-aminoalkyl metacrylate) having a pH of from 3 to 5 and a viscosity of at least 20 cp was employed.

[0021] In the comparison Examples, the same cationic (inorganic metal salt;AlCl₃) coagulant (6 cc) and then the same anionic polymer coagulant (16 cc) were added in this sequence.

[0022] Values of the amount of settlement (cc) after the lapse of five minutes and those of the water content (%) after 24 hours dewatering are shown in Table 1. As is clear from Examples 1 to 3 of the present invention, sludge treated in accordance with the present invention has an excellent settlement property and degree of dewatering. Settlement and coagulation of the sludge are rapid, and the remaining water content is significantly lower.

Example 4

[0023] In a manner similar and to a similar volume of test material to that in Examples 1 to 3, an anionic polymer coagulant (partially sulfomethylated polyacrylamide) (12 cc), a cationic polymer coagulant (polyethylene imine) (12 cc) and then another anionic polymer coagulant (poly-Na-acrylic acid) (6 cc) were added in this order to Specimen C.

[0024] The specimen showed an amount of settelement of 80 cc after the lapse of five minutes and a water content of 57% after the lapse of 24 hours. No turbidity of the supernatant liquid was observed.

[0025] According to the present invention, it is possible to coagulate sludge at a far higher efficiency with a far smaller dewatered load than by using conventional methods.

[0026] Spontaneous dewatering suffices for the purpose of dewatering sludge, and in the case of mechanical dewatering, a capacity of treatment about twice as high as the conventional one is possible. It is also not necessary to add any inorganic salt or pH adjusting reagent to the resultant for neutralisation purposes, thus permitting large-scale sludge treatment at a much lower cost.

Claims

1. A method of coagulating sludge, which comprises adding sequentially to the sludge an anionic polymer coagulant and a cationic polymer coagulant.

2. A method of coagulating sludge as claimed in claim 1, which comprises adding a further anionic polymer coagulant after addition of said cationic polymer coagulant.

3. A method as claimed in claim 1 or 2 wherein the anionic polymer coagulant comprises a 0.1% pure aqueous solution, a pH of 6 to 8 and a viscosity of at least 100 cp (25τC, a B-type rotary viscosimeter, 30 rpm).

4. A method as claimed in claim 3 wherein the anionic coagulant is an acrylamide polymer.

5. A method as claimed in any of claims 1 to 4 wherein the cationic polymer coagulant has a pH of 3 to 6.

6. A method as claimed in claim 5 wherein the cationic coagulant is an ester coagulant.

7. A method as claimed in any of claims 1 to 6 wherein the ratio of anionic to cationic polymer coagulant addition is 1.5:1.

8. A method of dewatering sludge coagulated by the method as claimed in any of claims 1 to 7, which comprises putting coagulated sludge in a cloth bag or a metal net bag.

9. A method of coagulating sludge as claimed in claim 1 substantially as hereinbefore described and with reference to any of the Examples.